If there are two queues, both allowed to use 100% of the actual resources in the cluster. Q1 and Q2 currently use 50% of actual cluster's resources and there is not actual space available. If we use current method to get headroom, CapacityScheduler thinks there are still available resources for users in Q1 but they have been used by Q2.
If the CapacityScheduelr has a hierarchy queue structure, it may report incorrect queueMaxCap. Here is a example
|(allowed to use up 80% of its parent)||(allowed to use 20% in minimum of its parent)|
|(50% of its parent)||(50% of its parent in minimum)|
When we calculate headroom of a user in L2LeafQueue2, current method will think L2LeafQueue2 can use 40% (80%*50%) of actual rootQueue resources. However, without checking L1ParentQueue1, we are not sure. It is possible that L1ParentQueue2 have used 40% of rootQueue resources right now. Actually, L2LeafQueue2 can only use 30% (60%*50%).